Radiation detector and radiation detection method

ABSTRACT

The present invention provides a radiation detector with high detection sensitivity. The radiation detector according to the present invention includes an Al 2 O 3  substrate, a Fe 2 O 3  thin film layered on the Al 2 O 3  substrate, a Ca x CoO 2  (where 0.15&lt;×&lt;0.55) thin film that is layered on the Fe 2 O 3  thin film and that has CoO 2  planes that are aligned inclined to the surface of the Al 2 O 3  substrate, a first electrode disposed on the Ca x CoO 2  thin film, and a second electrode disposed on the Ca x CoO 2  thin film in a position opposed to the first electrode in the direction in which the CoO 2  planes are aligned inclined.

TECHNICAL FIELD

The present invention relates to a radiation detector that utilizes ananisotropic thermoelectric effect and a radiation detection method usingthe same.

BACKGROUND ART

When a temperature difference is generated between both ends of athermoelectric conversion material, an electromotive force (a thermalelectromotive force) is generated in proportion to the temperaturedifference. The phenomenon that thermal energy is converted intoelectrical energy in a thermoelectric conversion material is known asthe Seebeck effect. The electromotive force V that is generated isexpressed as V=SΔT, where ΔT is a temperature difference and S is theSeebeck coefficient peculiar to the material.

In a thermoelectric conversion material that exhibits isotropic physicalproperties, the electromotive force generated by the Seebeck effect isgenerated only in the direction in which the temperature difference hasbeen generated. On the other hand, due to the inclined arrangement ofthe crystal axes, the thermoelectric conversion material that exhibitsanisotropy in its electrical transport properties generates anelectromotive force in the direction orthogonal to the direction inwhich the temperature difference has been generated. The electricaltransport properties denote the behavior of electrons and positive holeshaving electric charges that move in a substance. As described above,the phenomenon that due to the inclined arrangement of the crystal axesof the material, an electromotive force is generated in the directionthat is different from the direction in which the temperature differencehas been generated (a heat flow direction) is referred to as ananisotropic thermoelectric effect or an off-diagonal thermoelectriceffect.

FIG. 13 is a diagram of a coordinate system for explaining theanisotropic thermoelectric effect. As shown in FIG. 13, the crystal axesabc of the sample 101 are inclined to the spatial axes xyz. In thesample 101, when a temperature difference ΔT_(z) is applied in thedirection along the z axis, an electromotive force V_(x) is generated inthe direction orthogonal to the z axis, i.e. the direction along the xaxis. The electromotive force V_(x) is represented by Formula (1):

$\begin{matrix}{\left\lbrack {{Mathematical}\mspace{14mu} {Formula}\mspace{14mu} 1} \right\rbrack \mspace{464mu}} & \; \\{V_{x} = {\frac{l}{2d}\Delta \; {T_{z} \cdot \Delta}\; {S \cdot \sin}\; 2\alpha}} & (1)\end{matrix}$

where 1 denotes the width of the sample 101, d denotes the thickness ofthe sample 101, α denotes the inclination angle of the a-b plane to thesurface (the x-y plane) of the sample 101, and ΔS denotes the difference(the difference that occurs due to anisotropy) between the Seebeckcoefficient S_(c) in the c-axis direction and the Seebeck coefficientS_(ab) in the a-b in-plane direction.

Conventionally, a radiation detector using an inclined layered thin filmof YBa₂Cu₃O_(7-d) (hereinafter referred to as “YBCO”) has been proposedas a radiation detector that utilizes the anisotropic thermoelectriceffect (see, for example, Patent Literature 1). The inclined layeredthin film denotes a thin film that is layered on a substrate and thathas a structure in which the crystal axis is inclined to the surface ofthe substrate and a plurality of layers are layered together. The YBCOthin film has an anisotropic crystal structure in which a CuO₂ layerhaving electrical conductivity and Y and BaO layers that have insulationproperties are layered alternately along the c-axis direction. When theYBCO thin film is layered (inclined and layered) on a suitable substratesurface in such a manner that the c axis is inclined to the substratesurface, a similar system to that shown in FIG. 13 is formed. The CuO₂plane corresponds to the a-b plane shown in FIG. 13. When anelectromagnetic wave is incident on the surface of the YBCO thin filmthat has been inclined and layered as described above, a temperaturedifference is generated in the direction perpendicular to the surface ofthe YBCO thin film. As a result, an electromotive force is generated inthe direction parallel to the surface of the YBCO thin film by theanisotropic thermoelectric effect. Reading this electromotive forceallows the electromagnetic wave that has been incident on the surface ofthe YBCO thin film to be detected. A radiation detector using the YBCOthin film can detect an electromagnetic wave at a sensitivity ofapproximately 100 mV/K.

From Formula (1), the electromotive force V_(x) that is generated by theanisotropic thermoelectric effect is proportional to the difference ΔSthat occurs due to anisotropy of the Seebeck coefficient, the aspectratio 1/d of a sample, and a sine value of sin2α of an angle that istwice the inclination angle α. In the YBCO thin film, the difference ΔSis smaller than 10 μV/K, and the upper limit for maintaining theinclination angle α of the CuO₂ plane at a single angle is limited toapproximately 10 to 20° (see, for example, Non-Patent Literature 1 andNon-Patent Literature 2). Accordingly, the radiation detector thatincludes the YBCO thin film used therein cannot be said to havesufficiently high sensitivity for being used practically. In order toimprove the sensitivity of a radiation detector that includes aninclined layered thin film used therein, there are methods in which, forexample, a material with a larger difference ΔS is used and theinclination angle α of the thin film is brought close to 45 degrees asmuch as possible. Since the range of the inclination angle α in theinclined layered thin film depends on the combination of the thin filmmaterial and the substrate material on which the thin film material islayered, it is preferable that a suitable substrate material be selectedso that the inclination angle α can be controlled widely up to around45°.

Patent Literature 1 discloses a radiation detector in which a YBCO thinfilm partially doped with Pr is used. According to Patent Literature 1,the radiation detector has a sensitivity approximately twenty timeshigher than that of a radiation detector with a non-doped YBCO thin filmused therein. It is suggested that the reason for this is because theSeebeck coefficient of the YBCO thin film is increased by Pr doping.However, Non-Patent Literature 3 describes that in a Pr-doped YBCO thinfilm, the Seebeck coefficient increases in the direction of the a-bin-plane but remains unchanged in the c-axis direction. Furthermore,Non-Patent Literature 3 describes that the difference ΔS becomes smallerin the Pr doping range that is employed in the YBCO thin film used forthe radiation detector of Patent Literature 1. Therefore, using lightwith a wavelength (308 nm) that was different from light with awavelength of 248 nm used in Patent Literature 1, the response of thePr-doped YBCO thin film to light irradiation was actually measured bythe same experimental method as in Non-Patent Literature 3. As a result,it was confirmed that the Pr-doped YBCO thin film had a smallerelectromotive force that was generated by the anisotropic thermoelectriceffect.

The improvement in sensitivity of the radiation detector with thePr-doped YBCO thin film used therein is probably attributed to anincrease in absorption coefficient of the YBCO thin film with respect tolight with a wavelength of 248 nm due to Pr doping. That is, althoughthe radiation detector of Patent Literature 1 is highly sensitive tolight with a wavelength of 248 nm, it cannot be said that the detectionsensitivity is improved in other wavelength ranges.

[Prior Art Literature] [Patent Literature] [Patent Literature 1] JP8-247851 A [Non-Patent Literature]

[Non-Patent Literature 1]H. S. Kwok, J. P. Zheng, “Anomalousphotovoltaic response in YBa₂Cu₃O₇”, The American Physical Society,PHYSICAL REVIEW B, (1992), VOLUME 46, NUMBER 6, 3692

[Non-Patent Literature 2] Physica C 377 (2002) 26-35, Elsevier ScienceB. V. [Non-Patent Literature 3] 15th International Conference onThermoelectrics (1996), IEEE, pp. 494-498 DISCLOSURE OF INVENTION

The present invention is made with the above situation in mind and isintended to provide a radiation detector and radiation detection methodwith higher detection sensitivity.

The present inventors made various studies and found that theabove-mentioned object was achieved by the following present invention.That is, a radiation detector of the present invention includes an Al₂O₃substrate, a Fe₂O₃ thin film layered on the Al₂O₃ substrate, aCa_(x)CoO₂ (where 0.15<×<0.55) thin film that is layered on the Fe₂O₃thin film and that has CoO₂ planes that are aligned inclined to thesurface of the Al₂O₃ substrate, a first electrode disposed on theCa_(x)CoO₂ thin film, and a second electrode disposed on the Ca_(x)CoO₂thin film in a position opposed to the first electrode in the directionin which the CoO₂ planes are aligned inclined.

Furthermore, a radiation detection method of the present invention is aradiation detection method of detecting an electromagnetic wave using aradiation detector, wherein the radiation detector includes an Al₂O₃substrate, a Fe₂O₃ thin film layered on the Al₂O₃ substrate, aCa_(x)CoO₂ (where 0.15<×<0.55) thin film that is layered on the Fe₂O₃thin film and that has CoO₂ planes that are aligned inclined to thesurface of the Al₂O₃ substrate, a first electrode disposed on theCa_(x)CoO₂ thin film, and a second electrode disposed on the Ca_(x)CoO₂thin film in a position opposed to the first electrode in the directionin which the CoO₂ planes are aligned inclined, a thermal electromotiveforce is extracted that is generated between the first electrode and thesecond electrode according to a temperature difference generated in theCa_(x)CoO₂ thin film by an electromagnetic wave that is incident on theCa_(x)CoO₂ thin film, and the electromagnetic wave is detected accordingto the thermal electromotive force.

The present inventors studied various conditions and optimized them andthereby found that in a laminate having a three-layer structureincluding a Ca_(x)CoO₂ thin film, a Fe₂O₃ thin film, and an Al₂O₃substrate, it was possible to produce a Ca_(x)CoO₂ thin film with acrystal axis that was inclined considerably to the surface of the Al₂O₃substrate. According to the radiation detector that includes thelaminate having a three-layer structure, it is possible to increase theinclination angle of the crystal axis of the Ca_(x)CoO₂ thin film, whichis an inclined layered thin film. Therefore, it also is possible to makethe inclination angle approach 45°, and thereby the detectionsensitivity (an electromotive force) of the radiation detector can beincreased.

The present invention can provide a radiation detector and radiationdetection method with higher detection sensitivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an embodiment of the radiationdetector according to the present invention.

FIG. 2 is a diagram showing a θ-2θ scan XRD pattern of aCa_(x)CoO₂/Fe₂O₃/Al₂O₃-r thin film.

FIG. 3 is a diagram showing a θ-2θ scan XRD pattern of aCa_(x)CoO₂/Al₂O₃-r thin film.

FIG. 4 is a diagram showing a pole figure of theCa_(x)CoO₂/Fe₂O₃/Al₂O₃-r thin film.

FIG. 5 is a cross-sectional image of the three layers of aCa_(x)CoO₂/Fe₂O₃/Al₂O₃-r laminate.

FIG. 6 is a cross-sectional image of a vicinity of the Ca_(x)CoO₂/Fe₂O₃interface.

FIG. 7 is a high-resolution image of the inside of the Ca_(x)CoO₂ thinfilm.

FIG. 8 is a perspective view showing the configuration of a radiationdetector for measuring an electromotive force.

FIG. 9 is a graph showing change of electromotive force with timeaccording to incidence and cutoff of an electromagnetic wave.

FIG. 10 is a diagram showing the pole figure of aCa_(x)CoO₂/Fe₂O₃/Al₂O₃-n thin film.

FIG. 11 is a diagram showing the pole figure of aCa_(x)CoO₂/Fe₂O₃/Al₂O₃-S thin film.

FIG. 12 is a graph showing the relationship between sin2α and thevoltage obtained in the inclined alignment direction in radiationdetectors.

FIG. 13 is a diagram of a coordinate system for explaining theanisotropic thermoelectric effect.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a cross-sectional view of an embodiment of the radiationdetector according to the present invention. As shown in FIG. 1, theradiation detector 10 includes an Al₂O₃ substrate (a sapphire substrate)11, a Fe₂O₃ thin film 12 layered on the Al₂O₃ substrate 11, and aCa_(x)CoO₂ thin film 13 layered on the Fe₂O₃ thin film 12 as well as afirst electrode 14 and a second electrode 15 that are disposed on theCa_(x)CoO₂ thin film 13. In the Ca_(x)CoO₂ thin film 13, a deviation incomposition may occur depending on the production conditions, but it isacceptable as long as x satisfies 0.15<×<0.55.

The Ca_(x)CoO₂ thin film 13 is an inclined layered thin film and has alayer structure with CoO₂ layers and Ca, block layers that are layeredalternately. In the Ca_(x)CoO₂ thin film 13, the Seebeck coefficientS_(ab) in the in-plane direction of the CoO₂ planes 16 is different fromthe Seebeck coefficient S_(c) in the c-axis direction of the Ca_(x)CoO₂thin film 13, which is a direction perpendicular to the in-planedirection, and the Ca_(x)CoO₂ thin film 13 exhibits anisotropy.

In the Ca_(x)CoO₂ thin film 13, a plurality of CoO₂ planes 16 areinclined to the surface of the Al₂O₃ substrate 11 and are arranged inparallel with one another. The second electrode 15 is disposed to beseparated from the first electrode 14 in the electromotive-forceextracting direction 17. In other words, the electromotive-forceextracting direction is a direction in which the first electrode 14 andthe second electrode 15 are opposed to each other. Theelectromotive-force extracting direction 17 is perpendicular to the lineformed at an intersection of a CoO₂ plane 16 and the surface of theCa_(x)CoO₂ thin film 13 (the line in the direction perpendicular to theplane of the paper) and parallel with the surface of the Ca_(x)CoO₂ thinfilm 13 and is a direction in which the CoO₂ planes 16 are arrangedinclined. The CoO₂ planes 16 are inclined at an inclination angle α tothe electromotive-force extracting direction 17. Furthermore, the CoO₂planes 16 also are inclined at an inclination angle α to the surface ofthe Al₂O₃ substrate 11.

The radiation detector 10 has a three-layer structure composed of theCa_(x)CoO₂ thin film 13, the Fe₂O₃ thin film 12, and the Al₂O₃ substrate11. In a laminate with the three-layer structure, it is possible toproduce an inclined layered thin film (Ca_(x)CoO₂ thin film 13) with astructure in which the crystal axis is inclined considerably to thesurface of the Al₂O₃ substrate 11. Therefore, the inclination angle αcan be larger than that of the inclined layered thin film of aconventional radiation detector. In the radiation detector 10, theinclination angle α can be 10° to 80° and is preferably 25° to 65°. Thisallows a radiation detector 10 with high detection sensitivity to beobtained. As also is understood from Formula (1), it is particularlypreferable that the inclination angle α be 45° in the radiation detector10. In the radiation detector 10, the inclination angle α is allowed toapproach 45° further.

In the radiation detector 10, when an electromagnetic wave is incidenton the Ca_(x)CoO₂ thin film 13, the electromagnetic wave is absorbed bythe Ca_(x)CoO₂ thin film 13. This generates a temperature gradient inthe thin-film interplanar direction 18 in the Ca_(x)CoO₂ thin film 13.The thin-film interplanar direction 18 is perpendicular to the surfaceof the Ca_(x)CoO₂ thin film 13 and orthogonal to the electromotive-forceextracting direction 17. A temperature difference is generated in theCa_(x)CoO₂ thin film 13 and thereby an electromotive force is generatedin the electromotive-force extracting direction 17 in the Ca_(x)CoO₂thin film 13 by the anisotropic thermoelectric effect. The electromotiveforce thus generated is output to the outside through the firstelectrode 14 and the second electrode 15. The electromotive force outputthrough the first electrode 14 and the second electrode 15 is detectedand thereby the electromagnetic wave that has been incident on theCa_(x)CoO₂ thin film 13 can be detected.

The radiation detector 10 of the present invention can be produced bysequentially layering the Fe₂O₃ thin film 12 and the Ca_(x)CoO₂ thinfilm 13 on the Al₂O₃ substrate 11 and placing the first electrode 14 andthe second electrode 15 on the Ca_(x)CoO₂ thin film 13. The method oflayering the Fe₂O₃ thin film 12 and the Ca_(x)CoO₂ thin film 13 is notparticularly limited. For example, various methods can be used includingthose using vapor phase growth, such as a sputtering method, a vapordeposition method, a laser ablation method, and a chemical vapordeposition method, or those using growth from a liquid phase. Thethickness of the Fe₂O₃ thin film 12 and that of the Ca_(x)CoO₂ thin film13 are not particularly limited as long as both of them are equal to ormore than that of a unit cell layer. Specifically, they can beapproximately 50 nm to 200 nm. However, there is no problem even if thethicknesses are out of this range.

The inclination angle α of the CoO₂ planes 16 in the Ca_(x)CoO₂ thinfilm 13 is determined by the value of the inclination angle β formedbetween the surface of the Al₂O₃ substrate 11 and a (0001) plane 19 inthe Al₂O₃ substrate 11. Accordingly, in producing the radiation detector10, the Al₂O₃ substrate 11 having an inclination angle β correspondingto a desired value of the inclination angle α can be prepared. In thiscase, the inclination angle α is a value of approximately β±15° but thevalue of the inclination angle α may vary out of this range according tothe production conditions.

The first electrode 14 and the second electrode 15 are not particularlylimited as long as they are formed of materials with a high electricalconductivity. Specifically, a metal such as Cu, Ag, Mo, Al, Ti, Cr, Au,Pt, or In, a nitride such as TiN, or an oxide such as indium tin oxide(ITO) or SnO₂ can be used. Furthermore, a solder or a conductive pastemay be used to produce the first and second electrodes 14 and 15. Themethod of producing the first and second electrodes 14 and 15 on theCa_(x)CoO₂ thin film 13 is not particularly limited. Various methodssuch as application of a conductive paste, plating, thermal spraying,and solder joint with a solder can be used in addition to those usingvapor phase growth, such as a vapor deposition method and a sputteringmethod. The constituent material for the first electrode 14 and thesecond electrode 15 is preferably Cu, Ag, Au, or Al, more preferably Cu,Ag, or Au, and particularly preferably Cu or Ag.

The method of producing the radiation detector 10 is not particularlylimited to the above-mentioned methods as long as it is a method thatcan provide a three-layer structure composed of the Al₂O₃ substrate 11,the Fe₂O₃ thin film 12, and the Ca_(x)CoO₂ thin film 13 and that canplace the first and second electrodes 14 and 15 on the Ca_(x)CoO₂ thinfilm 13.

With respect to the radiation detector 10, the inclination angle α canbe controlled by controlling the inclination angle β of the (0001)planes 19 in the Al₂O₃ substrate 11 during the production thereof.Therefore, the inclination angle α can be controlled in a wide range.This makes it possible to obtain an inclination angle of the CoO₂ planesthat greatly exceeds the inclination angle of the CuO₂ planes in aconventional YBCO thin film, in the Ca_(x)CoO₂ thin film 13 having adifference ΔS approximately four times as large as that of theconventional YBCO thin film. Accordingly, it is possible to obtain aradiation detector whose performance greatly exceeds that of aconventional radiation detector with an inclined layered thin film. Thepresent invention promotes application of energy conversion between heatand electricity and therefore has a high industrial value.

Although the Ca_(x)CoO₂ thin film was used as the inclined layered thinfilm, it is expected that the same effect is obtained even when aSr_(x)CoO₂ thin film is used instead thereof.

EXAMPLES

Hereinafter, further specific examples of the present invention aredescribed.

Example 1 and Comparative Example

In Example 1, a Fe₂O₃ thin film was layered on an Al₂O₃-r planesubstrate whose surface had a (1-102) plane inclined at approximately57° to a (0001) plane and further, a Ca_(x)CoO₂ thin film was layered onthe Fe₂O₃ thin film. Thus, a laminate with a three-layer structure wasproduced. Hereinafter, the Ca_(x)CoO₂ thin film in the laminate isdescribed as a “Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-r thin film”. In this case, theinclination angle β is 57°. In producing thin films below,radio-frequency magnetron sputtering was used in all cases.

The Fe₂O₃ thin film (with a thickness of 100 nm) was produced on theAl₂O₃-r plane substrate (with a size of 10 mm×10 mm and a thickness of0.5 mm) using a Fe₂O₃ target. After the inside of a film forming chamberwas evacuated to 1.0×10⁻³ Pa or lower, the gas pressure inside thechamber was maintained at 1 Pa while an argon gas was introduced, andsputtering was carried out at a RF power of 100 W without heating with aheater.

For production of the Ca_(x)CoO₂ thin film (with a thickness of 150 nm),a target containing Ca and Co that were mixed together in such a manneras to have a molar ratio of 1:1 was used. After the inside of the filmforming chamber was evacuated to 1.0×10⁻³ Pa or lower, the Fe₂O₃/Al₂O₃-rlaminate was heated with a resistance heater while a mixed gas of argon(96%) and oxygen (4%) was introduced.

In order to select optimum conditions for producing the Ca_(x)CoO₂ thinfilm, the temperature of the Fe₂O₃/Al₂O₃-r laminate was varied from 400to 600° C., with the gas pressure being fixed at 5 Pa, as the filmforming conditions. The RF power used during sputtering was fixed at 100W. After deposition of the thin film, the mixed gas of argon (96%) andoxygen (4%) was introduced. While the gas pressure inside the chamberwas maintained at 5 Pa, it was cooled to room temperature over 60minutes. The cation composition ratio of the Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-rthin film was evaluated with an energy dispersive x-ray spectrometer. Asa result, the composition ratio of Ca and Co was approximately 1:2.Accordingly, x≈0.5.

Furthermore, as a comparative example, a Ca_(x)CoO₂ thin film waslayered on an Al₂O₃-r plane substrate and thus a laminate with atwo-layer structure was produced. Hereinafter, the Ca_(x)CoO₂ thin filmin the laminate is described as a “Ca_(x)CoO₂/Al₂O₃-r thin film”.

A Ca_(x)CoO₂ thin film (with a thickness of 150 nm) was produced on anAl₂O₃-r plane substrate (with a size of 10 mm×10 mm and a thickness of0.5 mm) using a target containing Ca and Co that had been mixed togetherin such a manner as to have a molar ratio of 1:1. After the inside ofthe film forming chamber was evacuated to 1.0×10⁻³ Pa or lower, theAl₂O₃-r plane substrate was heated with a resistance heater while amixed gas of argon (96%) and oxygen (4%) was introduced. In order toselect optimum conditions for producing the Ca_(x)CoO₂ thin film, thetemperature of the Al₂O₃-r plane substrate was varied from 400 to 600°C., with the gas pressure being fixed at 5 Pa, as the film formingconditions. The RF power used during sputtering was fixed at 100 W.After deposition of the thin film, the mixed gas of argon (96%) andoxygen (4%) was introduced. While the gas pressure inside the chamberwas maintained at 5 Pa, it was cooled to room temperature over 60minutes. The cation composition ratio of the Ca_(x)CoO₂/Al₂O₃-r thinfilm was evaluated with the energy dispersive x-ray spectrometer. As aresult, the composition ratio of Ca and Co was approximately 1:2.Accordingly, x≈0.5.

FIG. 2 is a diagram showing a θ-2θ scan X-ray diffraction (XRD) patternof a Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-r thin film. FIG. 2 indicates themeasurement result of the Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-r thin film obtainedwith the temperature being 500° C. in layering the Ca_(x)CoO₂ thin film.As shown in FIG. 2, besides the diffraction peaks derived from the(1-102) planes in the Al₂O₃-r plane substrate and the Fe₂O₃ thin film,one diffraction peak was observed at 2θ≈75°. This angle substantiallycoincides with the angle, at which a (022) diffraction peak of theCa_(x)CoO₂ thin film appears, which was determined according to theBragg condition. Therefor, it was suggested that in theCa_(x)CoO₂/Fe₂O₃/Al₂O₃-r thin film, the CoO₂ planes that were the (001)planes were inclined and layered with respect to the surface of theAl₂O₃ substrate.

FIG. 3 shows a θ-2θ scan XRD pattern of a Ca_(x)CoO₂/Al₂O₃-r thin film.FIG. 3 indicates the measurement result of the Ca_(x)CoO₂/Al₂O₃-r thinfilm obtained with the temperature being 500° C. in layering theCa_(x)CoO₂ thin film. As shown in FIG. 3, besides the diffraction peakderived from the (1-102) planes in the Al₂O₃-r plane substrate,diffraction peaks derived from the (001) planes (1=1, 2, 3, and 4) ofthe Ca_(x)CoO₂/Al₂O₃-r thin film were observed. In theCa_(x)CoO₂/Al₂O₃-r thin film, the (001) planes correspond to the CoO₂planes. Accordingly, in the Ca_(x)CoO₂/Al₂O₃-r thin film, it was foundthat the CoO₂ planes were layered in parallel with the surface of theAl₂O₃ substrate. In other words, the inclined layered structure was notobtained.

Next, in order to confirm the inclined layered structure of the CoO₂planes in the Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-r thin film that was suggested bythe θ-2θ scan XRD, the XRD pole figure measurement was carried out. Thepole figure measurement can provide information regarding theinclination of specific crystal planes to a substrate surface or thealignment direction thereof. With respect to the measurement conditions,the X-ray incident and detection angles (θ-2θ) are fixed at angles thatsatisfy the Bragg conditions, in the arrangement where crystal planes tobe measured are in parallel with a horizontal plane. In this state, thesubstrate plane was inclined (ψ=0 to 90°) from the horizontal directionand further was rotated (φ=0 to 360°) in an in-plane direction. Thescattered X-rays to be detected are reinforced by each other only whenthe target crystal planes are in parallel with the horizontal plane. Theinclination angle (the value of ψ) and the alignment direction (thevalue of φ) of the crystal planes can be obtained through themeasurement of intensity distribution of scattered light that isdetected, with ψ and φ being varied.

The XRD pole figure measurement was carried out with respect to theCa_(x)CoO₂/Fe₂O₃/Al₂O₃-r thin film, with 2θ being fixed at an angle atwhich a (001) diffraction peak of the Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-r thin filmappears. FIG. 4 shows the result. FIG. 4 shows that one diffraction peakhaving the maximum value at ψ≈60° and φ≈180° has appeared. Thisindicates that the (001) planes in the Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-r thinfilm are inclined at approximately 60° to the surface of the Al₂O₃substrate. Therefore, the inclination angle α is approximately 60°. Thisangle substantially coincides with the angle that is formed between a(011) plane and a (001) plane in the Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-r thin film.Furthermore, only one (001) diffraction peak observed in this polefigure indicated that the CoO₂ planes were inclined and layered in asingle direction in the Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-r thin film.

In order further to check the inclined layered structure of the CoO₂planes in the Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-r thin film, it was evaluated witha cross-sectional transmission electron microscope. FIG. 5 is across-sectional image of the three layers of a Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-rlaminate. FIG. 6 is a cross-sectional image of a vicinity of theCa_(x)CoO₂/Fe₂O₃ interface. FIG. 7 is a high-resolution image of theinside of the Ca_(x)CoO₂ thin film. As shown in FIGS. 5 to 7, a uniformstripe structure with an inclination of approximately 60° was observedclearly inside the Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-r thin film. A uniforminclined layered structure of the CoO₂ layer has been formed actually inthe Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-r thin film, and the inclination angle α isapproximately 60°. This coincides with the result obtained through thepole figure measurement. It can be understood from the above descriptionthat the Fe₂O₃ thin film is layered as a buffer layer on the Al₂O₃substrate and the Ca_(x)CoO₂ thin film is layered thereon, so that aCa_(x)CoO₂ thin film with the CoO₂ planes being inclined and layeredwith respect to the surface of the Al₂O₃ substrate can be produced.

FIG. 8 is a perspective view showing the configuration of a radiationdetector for measuring an electromotive force. As shown in FIG. 8, theradiation detector 20 includes an Al₂O₃ substrate 11, a Fe₂O₃ thin film12, and a Ca_(x)CoO₂ thin film 13 that are layered sequentially as wellas a first electrode pair 21 and a second electrode pair 22 that areplaced on the Ca_(x)CoO₂ thin film 13. The first electrode pair 21 is apair of electrodes disposed to be separated from each other along theinclined alignment direction 23 of CoO₂ planes 16. The second electrodepair 22 is a pair of electrodes disposed to be separated from each otheralong the direction perpendicular to the inclined alignment direction23. The inclined alignment direction 23 is identical to theelectromotive-force extracting direction. The first electrode pair 21and the second electrode pair 22 were disposed in such a manner that theintersection between the line segment extending between the respectiveelectrodes of the first electrode pair 21 and the line segment extendingbetween the respective electrodes of the second electrode pair 22 is thecenter position of each line segment. The second electrode pair 22 isused for checking whether an electromotive force is generated in thedirection perpendicular to the inclined alignment direction 23.Therefore, it does not need to be disposed in an actual radiationdetector.

A radiation detector with the configuration shown in FIG. 8 was producedusing a Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-r thin film. In aCa_(x)CoO₂/Fe₂O₃/Al₂O₃-r laminate, two pairs of electrodes composed ofAu to serve as the first electrode pair and the second electrode pairwere formed on the surface of the Ca_(x)CoO₂ thin film by the vacuumvapor deposition method. In each electrode pair, the width between therespective electrodes was set at 6 mm. In an actual radiation detector,the width between the electrodes is not limited to 6 mm and can beoptimized suitably according to the intended use and the installationlocation.

An electromagnetic wave generated from an infrared lamp (with awavelength of 800 to 2000 nm) was allowed to be incident on the surfaceof the radiation detector that had been produced, in such a manner thatthe spot diameter was 8 mm. Specifically, an electromagnetic wave at 480mW was output from the infrared lamp and was allowed to be incident onthe center of the Ca_(x)CoO₂ thin film surface, and then theelectromotive force V₁ generated in the inclined alignment direction andthe electromotive force V₂ generated in the direction perpendicular tothe inclined alignment direction were measured. FIG. 9 shows themeasurement result. FIG. 9 is a graph showing change of electromotiveforce with time according to incidence and cutoff of an electromagneticwave. When the electromagnetic wave output from the infrared lamp wasnot incident on the radiation detector, the electromotive forces V₁ andV₂ were not generated. When the infrared lamp was turned on and anelectromagnetic wave was allowed to be incident thereon, theelectromotive force V₁ increased rapidly and a value of approximately140 μV was indicated steadily. On the other hand, the electromotiveforce V₂ did not show a notable change. Thereafter, when the infraredlamp was turned off and thereby the electromagnetic wave was cutoff, theelectromotive force V₁ decreased rapidly to return to zero. On the otherhand, the electromotive force V₂ did not show a notable change.Accordingly, the direction in which an electromotive force is generatedin the radiation detector is the inclined alignment direction alone.Since the direction in which an electromotive force is generated dependson the inclined alignment direction of the CoO₂ planes, it can beunderstood that generation of the electromotive force V₁ results fromthe anisotropic thermoelectric effect.

The temperature difference ΔT_(z) that is generated between the upperand lower surfaces of the Ca_(x)CoO₂ thin film layer of the radiationdetector is estimated to be approximately 0.2 mK from Formula (1). Therespective values in Formula (1) are as follows: ΔS=35 μV/K, d=150 nm,1=6 mm, α=60°, and V_(x)=140 μV. Therefore, the detection sensitivity inthe inclined alignment direction reaches 600 mV/K. This is approximatelysix times as high as the detection sensitivity (100 mV/K) of a radiationdetector that includes a conventional YBCO inclined layered thin filmused therein.

Example 2

In Example 2, a Fe₂O₃ thin film and a Ca_(x)CoO₂ thin film were layeredsequentially on an Al₂O₃-n plane substrate whose surface had a (11-23)plane inclined at approximately 61° to a (0001) plane and thereby alaminate with a three-layer structure was produced in the same manner asin Example 1. Hereinafter, the Ca_(x)CoO₂ thin film in the laminate isdescribed as a “Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-n thin film”. In this case, theinclination angle δ of the Al₂O₃-n plane substrate is 61°.

Furthermore, in Example 2, a Fe₂O₃ thin film and a Ca_(x)CoO₂ thin filmwere layered sequentially on an Al₂O₃-S plane substrate whose surfacehad a (10-11) plane inclined at approximately 72° to a (0001) plane andthereby a laminate with a three-layer structure also was produced in thesame manner as in Example 1. Hereinafter, the Ca_(x)CoO₂ thin film inthe laminate is described as a “Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-S thin film”. Inthis case, the inclination angle β of the Al₂O₃-S plane substrate is72°.

With respect to the Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-n thin film and theCa_(x)CoO₂/Fe₂O₃/Al₂O₃-S thin film, the XRD pole figure measurement wascarried out. The results are shown in FIGS. 10 and 11. FIG. 10 showsthat in the Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-n thin film, the CoO₂ planes areinclined at approximately 75° to the surface of the Al₂O₃ substrate.Therefore, the inclination angle α of the Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-n thinfilm is approximately 75°. Furthermore, FIG. 11 shows that in theCa_(x)CoO₂/Fe₂O₃/Al₂O₃-S thin film, the CoO₂ planes are inclined atapproximately 80° to the surface of the Al₂O₃ substrate. Therefore, theinclination angle α of the Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-S thin film isapproximately 80°. Thus, it was confirmed that it was possible tocontrol the inclination angle α of the CoO₂ planes in the Ca_(x)CoO₂thin film in the Ca_(x)CoO₂/Fe₂O₃/Al₂O₃ laminate by controlling theinclination angle β at which the (0001) planes in the Al₂O₃ substratewere inclined to the surface of the Al₂O₃ substrate.

In the same manner as in Example 1, two Au electrode pairs were producedon each of the Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-n thin film and theCa_(x)CoO₂/Fe₂O₃/Al₂O₃-S thin film and thereby the radiation detectorswith the configuration shown in FIG. 8 were produced. Similarly withrespect to these radiation detectors, an electromagnetic wave wasallowed to be incident from an infrared lamp and the electromotive forceV₁ in the inclined alignment direction and the electromotive force V₂ inthe direction perpendicular to the inclined alignment direction weremeasured in the same manner as in Example 1.

In the radiation detectors including the Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-n thinfilm and the Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-S thin film of Example 2 that wereused therein, respectively, the electromotive forces V₁ and V₂ were notgenerated when no electromagnetic wave was incident thereon as in thecase of the radiation detector of Example 1. When the infrared lamp wasturned on and an electromagnetic wave was allowed to be incidentthereon, the electromotive force V₁ increased rapidly and a value ofapproximately 80 μV was indicated steadily in both the radiationdetectors. On the other hand, the electromotive force V₂ did not show anotable change in both the radiation detectors. Thereafter, when theinfrared lamp was turned off and thereby the electromagnetic wave wascutoff, the electromotive force V₁ decreased rapidly to return to zeroin both the radiation detectors. On the other hand, the electromotiveforce V₂ did not show a notable change in both the radiation detectors.

FIG. 12 is a graph showing the relationship between sin2α and thevoltage obtained in the inclined alignment direction in the radiationdetectors. In the graph, the vertical axis indicates the electromotiveforce obtained in the inclined alignment direction in each radiationdetector and the horizontal axis indicates sin2α. FIG. 12 shows theresults of measurements carried out by the radiation detectors includingthe Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-r thin film, the Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-nthin film, and the Ca_(x)CoO₂/Fe₂O₃/Al₂O₃-S thin film that were usedtherein, respectively. FIG. 12 shows that the electromotive force ischanged to a substantially linear shape with respect to sin2α asindicated in Formula (1). Furthermore, it also was confirmed from FIG.12 that the sensitivity of each radiation detector was improved as theinclination angle α approaches 45° as indicated in Formula (1).

INDUSTRIAL APPLICABILITY

The radiation detectors according to the present invention each haveexcellent radiation detection properties and can be used for detectingvarious objects involving irradiation of an electromagnetic wave in, forexample, a temperature sensor and a laser beam power meter.

1. A radiation detector, comprising: an Al₂O₃ substrate, a Fe₂O₃ thinfilm layered on the Al₂O₃ substrate, a Ca_(x)CoO₂ (where 0.15<×<0.55)thin film that is layered on the Fe₂O₃ thin film and that has CoO₂planes that are aligned inclined to the surface of the Al₂O₃ substrate,a first electrode disposed on the Ca_(x)CoO₂ thin film, and a secondelectrode disposed on the Ca_(x)CoO₂ thin film in a position opposed tothe first electrode in the direction in which the CoO₂ planes arealigned inclined.
 2. The radiation detector according to claim 1,wherein a thermal electromotive force is extracted that is generatedbetween the first electrode and the second electrode according to atemperature difference generated in the Ca_(x)CoO₂ thin film by anelectromagnetic wave that is incident on the Ca_(x)CoO₂ thin film, andthe electromagnetic wave is detected according to the thermalelectromotive force.
 3. The radiation detector according to claim 1,wherein the CoO₂ planes are inclined at an inclination angle α to thedirection in which the first electrode and the second electrode areopposed to each other, and the inclination angle α is 10° to 80°.
 4. Theradiation detector according to claim 1, wherein the first electrode andthe second electrode are composed of Cu, Ag, Au, or Al.
 5. A radiationdetection method of detecting an electromagnetic wave using a radiationdetector comprising, detecting an electromagnetic wave by a thermalelectromotive force that is extracted using a radiation detection,wherein the radiation detector comprises an Al₂O₃ substrate, a Fe₂O₃thin film layered on the Al₂O₃ substrate, a Ca_(x)CoO₂ (where0.15<×<0.55) thin film that is layered on the Fe₂O₃ thin film and thathas CoO₂ planes that are aligned inclined to the surface of the Al₂O₃substrate, a first electrode disposed on the Ca_(x)CoO₂ thin film, and asecond electrode disposed on the Ca_(x)CoO₂ thin film in a positionopposed to the first electrode in the direction in which the CoO₂ planesare aligned inclined, and the thermal electromotive force is extractedthat is generated between the first electrode and the second electrodeaccording to a temperature difference generated in the Ca_(x)CoO₂ thinfilm by an electromagnetic wave that is incident on the Ca_(x)CoO₂ thinfilm.
 6. The radiation detection method according to claim 5, whereinthe CoO₂ planes are inclined at an inclination angle α to the directionin which the first electrode and the second electrode are opposed toeach other, and the inclination angle α is 10° to 80°.